The present invention is related to switching systems, and more particularly to switching electronic circuits with reduced EMI.
Switching systems or switched mode systems are electronic circuits that employ transistors to regulate an electrical current or voltage by switching transistors on and off. A supply current or voltage is rapidly connected and disconnected from an output by the transistors, providing a reduced and controlled level of current or voltage at the output. For example, a number of well known designs exist for switched mode power converters and amplifiers, such as a bridge-tied load (10, FIG. 1) in a class D audio amplifier. However, switched mode systems are susceptible to causing electromagnetic interference (EMI) that can have undesirable effects on nearby electronic circuits.
Referring to FIG. 1, a typical bridge-tied load or H-bridge 10 includes a load 12 connected in an H configuration between a left high-side driver or switch 14 and low-side switch 16 and a right high-side switch 18 and low-side switch 20. In operation, current flows through the load 12 from left to right 22 when the left high-side switch 14 and right low-side switch 20 are closed and the right high-side switch 18 and left low-side switch 16 are open. Current flows through the load 12 from right to left when the right high-side switch 18 and left low-side switch 16 are closed and the left high-side switch 14 and right low-side switch 20 are open. A number of switching schemes are used to control the switches 14-20, including a pulse width modulation (PWM) scheme of type BD, wherein switch 14 and switch 16 are in opposite states determined by a comparison of an input signal to a triangular carrier signal, and switch 18 and switch 20 are in other opposite states determined by a comparison of the inverted input signal to that same triangular carrier signal. BD modulation is sometimes called three-level modulation because there are portions of every clock cycle when both switched output nodes 24 and 26 are either at VDD or at ground, effectively applying zero voltage across the load. The carrier frequency is usually several hundred kilohertz or higher for an audio amplifier, and the rapidly rising switching wave forms produce significant EMI radiation in the 30-1000 MHz range. This EMI is suppressed using LC filters between the switched output nodes (e.g., 24 and 26) and the load 12, extensive decoupling on the VDD node 30, and even faraday cages, although these techniques require external components to be connected to integrated circuits containing the switching systems, increasing the size, complexity and cost of the system.
Thus, for at least the aforementioned reason, there exists a need in the art for switching electronic systems with reduced EMI.